Clinical experience suggests that a corneal inlay (Kamra, AcuFocus) produces better visual and clinical results when implanted deeper than recommended by the product labeling. While the inlay’s instructions for use indicate the creation of a stromal pocket with a minimum depth of 200 μm for placement, implanting the device at 250 μm or deeper produces more stable refraction and improved visual acuity. (The target-depth for the intrastromal pocket should be between 200 and 250 μm.)
“Don’t be afraid to place [this inlay] deeper than the labeling,” said Phillip C. Hoopes Jr., MD, Hoopes Vision Center, Salt Lake City. “Our experience shows that your patients will reap a very clear clinical reward.”
Dr. Hoopes highlighted 12-month outcomes for the first 2 years of corneal inlay implantations performed at the Hoopes Vision Center. The retrospective case analysis study compared visual results for 75 inlays implanted less than 250 μm deep and 58 inlays implanted between 250 and 300 μm.
Patients in the two groups were similar. The average preoperative spherical equivalent for the shallow-implantation group was -0.45 D compared with -0.49 D for the deep-implantation group. Average patient age was 53 years.
“We began by seeing clinical results that were similar to the FDA study,” Dr. Hoopes said. “Some patients were having a hyperopic shift in their prescriptions, usually 3 to 6 months after surgery.
“During the first Kamra inlay user group meeting in 2015, surgeons began sharing their experience with implanting the inlay deeper,” he added. “The take-home message for us was that we began implanting the inlays at 250 μm or deeper.”
Deeper implantation was not a complete surprise, he continued.
The inlay was initially placed under a standard LASIK flap, but the shallow implantation tended to produce dryness and other problems. Surgeons began implanting the inlay deeper, eventually moving to a pocket procedure by the time the pivotal FDA study was designed and approved. At least one other inlay is routinely implanted at 300 μm.
“When we started to look at the comparative data, it is very clear that patients with deeper implantation of the Kamra had better clinical results,” Dr. Hoopes said.
Comparison with shallow implants
“If you look at changes in refraction, patients with the shallow implants had an average shift of +0.45 D between 3 and 6 months after surgery, whereas the group with the deeper implants had no change in their spherical equivalent during the first 12 months for which we collected data,” Dr. Hoopes said. “Their prescription was very stable with deeper implantation.”
Distance uncorrected visual acuity was good in both groups, which is not surprising given the implant is labeled for individuals who have good uncorrected distance vision, he noted. The results were strikingly different for uncorrected near vision.
Before implantation, only 5% of patients were 20/25 or better and 40% were 20/40 or better. At 3 months after surgery, 34% of patients with shallow implants were 20/25 or better and 81% were 20/40 or better versus 45% and 90% of patients with deep implants. Fully 81% of shallow-pocket patients were J5 or better compared with 90% for deep-pocket patients.
By 6 months, the gap had opened to 22% versus 42% for 20/25 and 60% versus 84% for 20/40. Reflecting the shift in spherical equivalent, only 60% of shallow-pocket patients were J5 or better compared with 84% of deep-pocket patients.
At 12 months, the gap was 27% versus 59% for 20/25 and 77% versus 89% for 20/40.
“We saw a clear difference in visual performance at the greater implantation depth,” Dr. Hoopes said.
“The theory is that there are fewer keratocytes the deeper you go in the cornea, less cellular matrix to disrupt with surgery,” he said. “So you have less wound healing and less reactivity to the inlay within the cornea. The more shallow the implant, the more keratocytes and the greater the opportunity for inflammation, wound-healing complications, and other physiologic changes that can affect clinical outcomes.”
Debate over the ideal implantation depth continues, Dr. Hoopes added.
The discussions are similar to those surrounding the residual stromal bed for LASIK with some surgeons recommending 250 μm, whereas others see advantages in 300 μm.
Dr. Hoopes noted his practice has several patients with inlays as deep as 300 μm.
The ideal implantation depth depends at least in part on the preoperative corneal thickness. The Kamra advisory board currently recommends implantation at 40% of the cornea depth, he noted.
“If you implant at the [minimum] depth on the label, you tend to see a hyperopic change in refraction beyond 3 months,” he said. “When you implant deeper than 250 μm, patients have refractive stability for at least the first year and better visual acuity than a shallow implant. There is a clear difference in clinical performance with deeper implantation.”